This invention relates to mixers of the type employed in sanitary processing of pharmaceuticals, cosmetics, or other products that are to be kept safe for human use, and is more specifically related to an improvement in dual agitator mixers that permits cleaning in place of all interior surfaces.
Coaxial, dual-agitator and single-agitator mixers are employed in industries such as pharmaceuticals, cosmetics, and food products where the ingredients to be mixed and the resulting product need to be kept sanitary or in some cases sterile.
In practice, these mixers are in the form of a large tank or similar vessel, which may have a capacity from about twenty gallons up to tens of thousands of gallons, usually formed of stainless steel, either with one agitator or with two internal agitators that rotate in opposite directions. Usually these are top-entering mixers, with coaxial vertical drive shafts that penetrate the center of the closed top of the vessel, with a lower-speed outer scraper agitator and a higher-speed inner agitator. The drive shafts are collocated with the axis of the tank or vessel, so that the agitators are centered in the vessel. The two agitators have respective drive shafts, driven by independent gearmotors, which can be electric or hydraulic, with an upper gearmotor driving the inner agitator and the lower gearmotor driving the outer or scraper agitator.
In mixers of this arrangement, the inner drive shaft that drives the inner agitator descends through a hollow outer shaft that drives the outer agitator. The outer shaft ends a short distance below the point where it enters the mixing vessel, with the inner shaft continuing on downward. This arrangement requires that there be two rotary seals: one seal where the outer shaft penetrates the top or head of the vessel, and a second rotary seal where the inner shaft exits the lower end of the hollow outer shaft. This design closes off and seals the hollow annular space between the inner shaft and the outer shaft.
A number of issues arise where the second seal, or annular space seal, resides within the tank or vessel. The inner seal is difficult to clean, especially when using clean-in-place or C.I.P. techniques. The seal may also fail, resulting in pieces of the seal falling into the product. Servicing the seal requires a person to enter the confined space inside the vessel, which leads to safety concerns for the user. In this typical arrangement the inner seal is often three feet or more away from the associated gearmotor, which makes the seal more susceptible to increased shaft run-out, carbon wear, which may contaminate the product, and premature seal failure. Also, any oil leakage from the upper gear motor will tend to run into the annular space where it may escape past the inner seal and enter the product in the vessel.
Where a clean-in-place or C.I.P. system is used to clean and sanitize the vessel and agitators between uses, only the agitators and the outer surfaces of the drive shafts can be exposed to the spray of the C.I.P. fluid, and it has not been possible to spray or flow C.I.P. fluid into the annular space. Even if an inner seal for the annular space is located outside the vessel, there is currently no satisfactory technique for cleaning the annular space and, despite that space being out of direct contact with the product itself, the annular space is continuously exposed to various vapors, and presents a significant cleaning concern.
Traditional C.I.P. cleaning of agitators and seal areas is carried out using in-tank spray devices. Some seals have provisions for flushing (which is not effective cleaning), it would be preferable to have a rotating spray of the C.I.P. fluid sprayed directly onto the seal faces and seal glands. Seal design is an industry concern, especially in terms of efforts to make the seals more sanitary. However, current modes of internal cleaning do not reach areas such as seals between inner and outer drive shafts, nor do they reach the annular spaces between inner and outer drive shafts.
According to an aspect of this invention, a sanitary mixer arrangement comprises a vessel with one or more agitators and associated drives. The vessel contains the ingredients that are to be mixed, and has a side wall and a closed top, with the mixer vessel having a vertical axis. In a dual-agitator mixer, an outer or scraper agitator and an inner agitator are driven respectively by an outer vertical tubular drive shaft and inner vertical drive shaft coaxial with the outer drive shaft. The drive shafts penetrate an opening at the closed top of the vessel and descent along the axis of the vessel. An outer seal is positioned at the opening in the closed top of vessel around the outer drive shaft and permitting rotation of the outer drive shaft. A lower drive arrangement rotates the outer drive shaft and is positioned above the closed top of the vessel. An upper drive arrangement drives the inner drive shaft and is positioned above the lower drive arrangement.
The outer drive shaft has an upper terminus beneath the upper drive arrangement. The inner drive shaft extends up above that terminus. The outer drive shaft and said inner drive shaft define an annular space between them that descends down from the terminus and through the closed top of the vessel, with the annular space being open into the interior of the vessel. The inner seal is mounted in said annular space at the terminus of said outer drive shaft, permitting rotation of the inner drive shaft relative to the outer drive shaft.
A central bore is drilled along the axis of the inner drive shaft from an upper end thereof down to a lower end position below the inner seal and above the closed top of the vessel.
A permanent rotary fitting at the upper end of the inner drive shaft permits the introduction of a cleaning fluid under pressure into the central bore. The cleaning fluid passes through the bore and the at least one radial passage into the annular space to clean accumulations from within the annular space and exhaust it into the interior of vessel.
Favorably, the outer agitator has a plurality of scraper vanes that sweep past the interior surface of the vessel side wall.
In a preferred embodiment, the inner drive shaft is solid below the lower end position of said central bore.
Several radial passages extend from the lower end position of the central bore to said annular space, to conduct the fluid from the central bore into the upper end of the annular space. That is, the upper part of the inner shaft is “rifle bored” with an axial passage, and at the bottom end of this passage the shaft is cross-bored so that there is a series of holes to create a rotary spray device with the cross-bored holes being strategically placed to wash the underside of the inner seal and to flush out the annular space.
A rotary union installed at the top end of the inner shaft allow a C.I.P. pipe to be permanently connected, so that the annular space can be flushed, automatically, every time the vessel C.I.P. system is run. The C.I.P. system can be hard-piped to the rotary union, so that the cleaning of the annular space occurs without any operator interaction. This results in the elimination of any cleaning concern about the annular space, and eliminates the potential for carbon dust to build up on the seal faces and fall to into the product over time. This arrangement also allows for service without need for an expensive confined-space entry.
In the described embodiment, the outer drive shaft has at least one cleaning slot extending therethough at a position below the closed top of the vessel and above a lower end of the outer drive shaft. An in-tank spray device within said vessel and near the top or head directs a spray of the C.I.P. fluid towards outer drive shaft to spray cleaning fluid at the cleaning slot or slots. This can provide additional cleaning of the lower part of the annular space and wash out any accumulated carbon or other possible contaminant.
The through-the-shaft-cleaning design can likewise be applied to a single-mixer design where the fluid is supplied into an axial bore in a single drive shaft to clean the shaft seal faces and also to clean the annular space around the shaft where it enters the tank head, especially in those cases where the space is long, e.g., where the tank head is insulated.
As is the conventional practice, the C.I.P. cleaning solution is made up as an aqueous solution of a caustic material, such as potassium hydroxide (KOH). At the end of the caustic phase of the cleaning process, the caustic solution and any entrained solids are discharged to a drain, and the solution is considered waste. A caustic recovery system may fitted to the C.I.P. system.
While the invention has been described with reference to a few selected embodiments, it should be recognized that the invention is not limited to those precise embodiments. Rather, many modifications and variations will be apparent to persons skilled in the art without departing from the scope and spirit of this invention, as defined in the appended claims.